Optical drive capable of replaying optical carriers with high birefringence

ABSTRACT

The invention relates to an optical drive for reading information from an optical disk ( 1 ) with a high birefringence. A radiation beam ( 5 ) has a modulated read power level of frequency F 1 . A polarizing beam splitter (PBS;  6 ) guides the reflected beam ( 8 ) from the disk ( 1 ) towards a photo detector ( 10 ) that outputs optical response signals (RS). The optical response signals (RS) are converted to optical response parameters (RP) that are compared with a predetermined optical response parameter reference values (RP_ref). If a sufficient deviation (Δ) is present it is indicative of an optical feedback from the beam splitting means (PBS;  6 ) towards to the radiation source ( 4 ). The processing means ( 52 ) can then change the first modulator frequency (F 1 ) of the radiation beam ( 5 ) to a second modulator frequency (F 2 ) so as to decrease the optical feedback. The invention provides a relatively simple yet effective way of compensating for a high birefringence value on an optical carrier wherefrom information is replayed.

FIELD OF THE INVENTION

The present invention relates to an optical drive for readinginformation from an associated optical carrier capable of replayingoptical carriers with high birefringence. The invention also relates toa corresponding method for operating an optical drive and correspondingprocessing means for controlling an optical drive.

BACKGROUND OF THE INVENTION

Optical recording and replaying on optical carriers such as opticaldisks of the CD (compact disk), DVD (digital versatile disc) or BD(Btu-Ray disk) format can in general be performed with optical drivesutilizing polarizing optics or non-polarizing optics.

Optical drives with non-polarizing optics have a relatively simpledesign and are accordingly robust in their performance. Such opticaldrives are for example not influenced by birefringence from the opticaldisk. However, for optical drives with non-polarizing optics the powerdelivered to the optical disk and in turn reflected to the detectionphotodiodes is more difficult to control. This problem is particularlyimportant if information is to be written on the optical disk as powercontrol is essential for reliable recording on an optical disk. For thisreason most optical drives apply polarizing optics to facilitate moreefficient power control in the optical system consisting of the opticaldrive and optical carrier.

Optical drives with a laser as irradiation source inherently has apolarized light source and appropriate optical elements are thenprovided for defining and controlling the optical path with respect tothe polarized light. Optical elements include polarizing beam splitters,half wavelength plates, quarter wavelength plates etc. The opticalcarrier or disk itself can negatively influence the polarized lightimpinging on the optical disk if the birefringence of the disk is abovethe specified level for the optical disk in question. The birefringenceof the optical disk can thereby introduce an unacceptably highdistortion of the polarized, reflected light from the optical diskresulting in a possible optical power loss in the return path towardsthe photo detection means of the optical drive. Consequently, theread-out signal from the optical disk can be of a reduced quality oreven impossible to decode resulting in a complete failure of the opticaldrive.

The birefringence of the optical disk typically originates from theproduction process where short cycle times and/or fast cooling times inthe moulding process can introduce an anisotropic behavior in theoptical parameters (i.e. the refractive index) of the moulded opticaldisk. Normally, the optical disk is injection moulded in polycarbonate(PC), the injection moulding can introduce shrinkage, flow lines, andinclusions in the substrate. Usually, the birefringence is more severenear the outer diameter. During recording/replaying of the optical disk,the fast rotation in the optical drive can also increase the strain inthe optical disk and in turn aggravate the birefringence of the opticaldisk.

The problem of unacceptably high birefringence of optical disks iscommonly known in art and a range of technical solutions are available.It should be mentioned that the problem with optical disks havingbirefringence values beyond the appropriate specifications, so-called“out-of-spec” disks, is generally an increasing phenomena due to thehighly competitive trend towards low-cost optical disks.

US patent application 2005/259553 discloses a technical solution where abirefringence correcting element (see element 5 a in FIG. 1 and FIG. 14)is inserted in the optical path of the optical drive in order to correctfor birefringence in the optical disk. The correcting element comprisesa material with mono-axial refractive index anisotropy and thecorrecting element is located before an objective lens of the opticaldrive. In one embodiment, the correcting element is divided into fourregions by two straight lines passing an optical axis and intersectingeach other at right angles. Each of the four regions is radially dividedinto four sub-regions by three circles centered at the optical axis. Bythis optical design the phase difference that is produced when light isreflected by the birefringent disk can be offset or cancelled by thephase difference that is produced when the light is transmitted throughthe correcting element. However, the number of regions has to be quitehigh in order to provide a complete or near-complete correcting effect.This is therefore a solution which is expensive to implement intomass-scale production of optical drives.

Hence, an improved optical drive would be advantageous, and inparticular a more efficient and/or reliable optical drive would beadvantageous.

SUMMARY OF THE INVENTION

Accordingly, the invention preferably seeks to mitigate, alleviate oreliminate one or more of the above mentioned disadvantages singly or inany combination. In particular, it may be seen as an object of thepresent invention to provide an optical drive that solves the abovementioned problems of the prior art with optical carriers havingrelatively high birefringence values.

This object and several other objects are obtained in a first aspect ofthe invention by providing an optical drive for reading information froman associated optical carrier, the optical drive comprising:

a radiation source capable of emitting a radiation beam having a readpower level modulated with a first modulator frequency (F1), saidradiation beam being optically arranged to impinge on the associatedoptical carrier,

polarizing beam splitting means (PBS) arranged for guiding a reflectedbeam from the carrier towards photo detection means, said photodetection means being capable of outputting optical response signals(RS), and

processing means for processing said optical response signals (RS)received from the photo detection means into one or more opticalresponse parameters (RP), said processing means comprising comparisonmeans arranged for monitoring if a optical response parameter (RP) isdeviating from a pre-determined optical response parameter referencevalue (RP_ref), said deviation being indicative of an optical feedbackfrom the beam splitting means (PBS) towards to the radiation source, theprocessing means further being adapted for changing the first modulatorfrequency (F1) of the radiation beam to a second modulator frequency(F2) so as to decrease the optical feedback.

The invention is particularly, but not exclusively, advantageous forobtaining an optical drive that provides a relatively simple yeteffective way of compensating for a relatively high birefringence valueon an optical carrier wherefrom information is replayed. The opticaldrive compares one or more optical response parameters (RP) withreference values so as to obtain an indication of the birefringence ofthe optical carrier being replayed, and accordingly the optical drive iscapable of modifying the frequency of the radiation beam applied forreading the optical carrier so as to lower the optical feedbackresulting from the birefringence of the optical carrier being read.

The invention is furthermore relatively easy to implement in existingoptical drive technology as most optical drives already have a readinglaser beam that is modulated with a fixed frequency. For electromagneticshielding purposes (EMC), this frequency has hitherto been kept constantduring operation of the optical drive i.e. the optical drive has beendesigned for a certain fixed modulator frequency. The technical problemwith huge birefringence of optical carriers has then been solved by e.g.inserting a correcting optical element in the optical drive as in USpatent application 2005/259553, or other corrective means or methodsknown in the art. Preliminary testing performed by the applicant showsthat the majority of the prior art solutions commercially available arealso not capable of dealing with relatively large birefringence valuesas compared to the present invention.

It is to be understood that the meaning of the term “deviation” in thecontext of the present invention, i.e. the deviation of the opticalresponse parameter (RP) from a pre-determined optical response parameterreference value (RP_ref), can include absolute deviation within ameasurement uncertainty and relative deviation within a measurementuncertainty. The deviation can also be understood as a deviation beyonda certain threshold, preferably on a time-averaged basis, in order todiscriminate random noise and isolated errors or events.

In one embodiment, the optical response signals (RS) may representsinformation read from the optical carrier e.g. the optical responsesignal (RS) could be the high frequency (HF) signal. Thereby,irradiation source used for reading information is applied for measuringthe birefringence of the optical carrier being read. The opticalresponse parameter (RP) may then be chosen from the group consisting of:an address locating a position on the optical carrier, an uncorrectableerror in an information sequence read from the optical carrier, and anasymmetry value (beta) measured from the optical response signals (RS)from the optical carrier. The advantage is that these parameters arealready available for other purposes and accordingly the presentinvention may relatively easy be implemented. Moreover, the asymmetryvalue, or equivalents thereof, is a relatively good measure of thebirefringence of the optical carrier being replayed.

In alternative embodiment, the optical drive may have comprise anauxiliary radiation source adapted for emitting an auxiliary radiationbeam, said beam being optically arranged for impinging on the opticalcarrier and resulting in a reflected beam towards the photo detectionmeans. In this embodiment, the birefringence of the optical carrier canbe assessed without using the irradiation source applied for readinginformation. Thus, a dedicated light source for measuring birefringencecan be installed in the optical drive. Therefore, in this particularembodiment the photo detection means can possible not be applied inreading information from the optical carrier, and accordingly theoptical drive may then further comprise auxiliary photo detection meansfor detection of reflected radiation representing information read fromthe optical carrier.

In one embodiment often implemented, the second modulator frequency (F2)may be higher than the first modulator frequency (F1). Thus, the firstmodulator frequency (F1) can be increased in selected steps of 5%, 10%,15%, or 20%. Alternatively, absolute increments may be applied, such as5, 10, 15, 20, or 25 MHz. It should be noted that in general, the powerdissipation is increasing for increasing modulator frequency due to thecapacitive coupling of semiconductor lasers. Thereby, this embodiment isnot a preferred choice for power considerations.

Additionally or alternatively, the processing means may be arranged forincreasing the modulator frequency (F1; F2) in a feedback loop so tominimize the deviation of an optical response parameter (RP) from apre-determined optical response parameter reference value (RP_ref).Thereby, a quite effective way of iteratively compensating a largerinterval of birefringence values is provided.

In order to manipulate the radiation beam with respect to power, thebeam may have a substantially square-wave power profile. Due to the veryhigh frequency some deviations from square-wave will occur in realimplementations. The teaching of the present invention is not limited tothis power profile but may include a variety of power profiles such assinusoidal profiles, multi-level stepping profiles, etc. It is howevercrucial that the power is periodically effectively turned off so as todecrease the optical feedback, as it will be explained in more detailbelow.

In one embodiment, the radiation beam may be optically arranged forpassing the polarizing beam splitting means (PBS) on the optical pathtowards the optical carrier as this provides a simple optical path forthe optical drive.

In a second aspect, the invention relates to processing means forcontrolling an associated optical drive for reading information from anassociated optical carrier, the optical drive comprising:

a radiation source capable of emitting a radiation beam having a readpower level modulated with a first modulator frequency (F1), saidradiation beam being optically arranged to impinge on the associatedoptical carrier, and

polarizing beam splitting means (PBS) arranged for guiding a reflectedbeam from the carrier towards photo detection means, said photodetection means being capable of outputting optical response signals(RS),

the processing means being adapted for processing said optical responsesignals (RS) received from the photo detection means into one or moreoptical response parameters (RP), said processing means comprisingcomparison means arranged for monitoring if an optical responseparameter (RP) is deviating from a pre-determined optical responseparameter reference value (RP_ref), said deviation being indicative ofan optical feedback from the beam splitting means (PBS) towards to theradiation source, the processing means further being adapted forchanging the first modulator frequency (F1) of the radiation beam to asecond modulator frequency (F2) so as to decrease the optical feedback.

In a third aspect, the invention relates to a method for operating anoptical drive for reading information from an optical carrier, themethod comprising the steps:

emitting a radiation beam having a read power level modulated with afirst modulator frequency (F1), said radiation beam being opticallyarranged to impinge on the optical carrier,

guiding by polarizing beam splitting means (PBS) a reflected beam fromthe carrier towards photo detection means, said photo detection meansbeing capable of outputting optical response signals (RS), and

processing said optical response signals (RS) received from the photodetection means into one or more optical response parameters (RP),

monitoring if an optical response parameter (RP) is deviating from apre-determined optical response parameter reference value (RP_ref), saiddeviation being indicative of an optical feedback from the beamsplitting means (PBS) towards to the radiation source, and

changing the first modulator frequency (F1) of the radiation beam to asecond modulator frequency (F2) so as to decrease the optical feedback.

In a fourth aspect, the invention relates to a computer program productbeing adapted to enable a computer system comprising at least onecomputer having data storage means associated therewith to operating anoptical drive according to the third aspect of the invention.

This aspect of the invention is particularly, but not exclusively,advantageous in that the present invention may be implemented by acomputer program product enabling a computer system to perform theoperations of the second aspect of the invention. Thus, it iscontemplated that some known optical drive may be changed to operateaccording to the present invention by installing a computer programproduct on a computer system controlling the said optical recordingapparatus. Such a computer program product may be provided on any kindof computer readable medium, e.g. magnetically or optically basedmedium, or through a computer based network, e.g. the Internet.

The first, second, third and fourth aspect of the present invention mayeach be combined with any of the other aspects. These and other aspectsof the invention will be apparent from and elucidated with reference tothe embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be explained, by way of example only,with reference to the accompanying Figures, where

FIG. 1 is a schematic diagram of an optical drive according to thepresent invention,

FIG. 2 is a schematic illustration of optical feedback in an opticaldrive,

FIG. 3 is a simplified diagram of an optical drive according to thepresent invention,

FIG. 4 is a simplified graph showing the power output of an irradiationsource,

FIG. 5 is a schematic diagram of the optical path for an optical drivereading an optical carrier with no birefringence,

FIG. 6 is a schematic diagram of the optical path for an optical drivereading an optical carrier with birefringence,

FIG. 7 is a graph showing the birefringence across an optical carrier,

FIG. 8 is a flow-chart of a method according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1 shows an optical replaying/recording apparatus or an opticaldrive and an optical information carrier 1. The carrier 1 is fixed androtated by holding means 30.

The optical carrier 1 comprises a material suitable for recordinginformation by means of a radiation beam 5. The recording material may,for example, be of the magneto-optical type, the phase-change type, thedye type, metal alloys like Cu/Si or any other suitable material.Information may be recorded in the form of optically detectable effects,also called “marks” for rewriteable media and “pits” for write-oncemedia, on the optical carrier 1.

The optical apparatus, i.e. the optical drive, comprises an optical head20, sometimes called an optical pick-up (OPU), the optical head 20 beingdisplaceable by actuation means 21, e.g. an electric stepper motor, alinear motor, or a DC motor. The optical head 20 comprises a photodetection system 10, a laser driver device (LDD), a radiation source 4,a beam splitter 6, an objective lens 7, and lens displacement means 9capable of displacing the lens 7 both in a radial direction of thecarrier 1 and in the focus direction.

The function of the photo detection system 10 is to convert radiation 8reflected from the carrier 1 into electrical signals. Thus, the photodetection system 10 comprises several photo detectors, e.g. photodiodes,charged-coupled devices (CCD), etc., capable of generating one or moreelectric output signals. The photo detectors are arranged spatially toone another and with a sufficient time resolution so as to enabledetection of error signals, i.e. focus error FE and radial trackingerror RE. The focus error FE and radial tracking error RE signals aretransmitted to the processing means 50 where a commonly knownservomechanism operated by using PID control means(proportional-integrate-differentiate) is applied for controlling theradial position and focus position of the radiation beam 5 on thecarrier 1.

The radiation source 4 for emitting a radiation beam or a light beam 5can for example be a semiconductor laser with a variable power, possiblyalso with variable wavelength of radiation. Alternatively, the radiationsource 4 may comprise more than one laser. In the context of the presentinvention the term “light” is considered to comprise any kind ofelectromagnetic radiation suitable for optical recording and/orreproduction, such as visible light, ultraviolet light (UV), infraredlight (IR), etc. The radiation source 4 is controlled by a laser driverdevice (LDD) 22 arranged for supplied the radiation source 4 with theappropriate time-dependent current. The driver device 22 is in turncontrolled from the processing means 50. The processing means 50controls the driver device 22 to emit the radiation beam 5 with amodulator frequency F1 or F2 as indicated in FIG. 1.

The processing means 50 also receives and analyses signals from thephoto detection means 10, in particular optical response signals RS. Theprocessing means 50 can also output control signals to the actuationmeans 21, the radiation source 4, the lens displacement means 9, and therotating means 30, as schematically illustrated in FIG. 1. Similarly,the processing means 50 can receive data to be written, indicated at 61,and the processing means 50 may output data from the reading process asindicated at 60. While the processing means 50 has been depicted as asingle unit i.e. a processor in FIG. 1, it is to be understood thatequivalently the processing means 50 may be a plurality ofinterconnecting processing units positioned in the optical recordingapparatus, possibly some of the units may be positioned in the opticalhead 20.

FIG. 2 is a schematic illustration of optical feedback in an opticaldrive with an optical carrier 1 showing only a few selected opticalelements but nevertheless showing the basic principle of opticalfeedback. The radiation source 4, i.e. the laser, can be seen as a boxwith a specific electromagnetic (EM) standing wave pattern. A part ofthis wave 5 travels towards the carrier 1 and is reflected by thecarrier 1 back as radiation 8. This path of radiation 5 and 8 can alsobe seen as a specific electromagnetic (EM) standing wave pattern. Inoptical storage, this is an unwanted component on the interface TNT ofthe radiation source 4 and should be reduced as much as possible by thelight-path, e.g. by introducing beam splitters. Thus, when the reflectedlight 8 has a non-vanishing component on the interface INT of theradiation source 4 a rather complex process takes place. This is knownas optical feedback.

When a data pattern is present on the carrier 1 there will be aninstantaneous non-linear shift of signal level at the interface INT asis well known in the art of optical storage. This means that there willbe a shifting of the shorter run-lengths within the EFM pattern. This isalso known as an asymmetry (or beta) shift. Thus, in an extremesituation the result of this non-linear multiplicative process is thatthe short run lengths (2T, 3T etc. depending on the carrier format) willactually disappear making it very difficult or impossible to decode theinformation stored on the optical carrier 1. The optical feedback isalways present under real conditions and will result in a coherentprocess on the interface INT.

FIG. 3 is a simplified diagram of an optical drive comprising aradiation source 4 capable of emitting a radiation beam 5 having a readpower level modulated with a first modulator frequency F1. The radiationbeam 5 is optically arranged to impinge on optical carrier 1, thecarrier 1 having a significant birefringence, i.e. Δn≠0.

Polarizing beam splitting means (PBS) 6 are arranged for guiding thereflected beam 8 from the carrier 1 towards photo detection means 10.The photo detection means 10 is capable of outputting optical responsesignals RS from the optical carrier 1 to the processing means 50.

The processing means 50 is adapted for processing by sub-processor 53the optical response signals RS received from the photo detection means10 into one or more optical response parameters RP. The processing means50 further comprises comparison means 51 arranged for monitoring if anoptical response parameter RP is deviating from a pre-determined opticalresponse parameter reference value RP_ref. The deviation is anindication of an optical feedback from the beam splitting means (PBS) 6towards to the radiation source 4. If the deviation, indicated by thesymbol Δ (Greek delta), is of a sufficient magnitude and/or character,the processing means 50 is further adapted for changing the firstmodulator frequency F1 of the radiation beam to a second modulatorfrequency F2 by a sub-processor 52 controlling the laser driver device22. The processor 50 is thereby capable of decreasing the opticalfeedback by varying the modulator frequency from F1 to F2, normally byincreasing the modulator frequency, i.e. F2 being larger than F1.

The optical response signals RS can represent information read from theoptical carrier, e.g. the optical response signal could be the highfrequency (HF) signal in one embodiment. The optical response parametersRP can then be e.g. an address locating a position on the opticalcarrier 1. If the address is in an erroneous format or otherwiseunreadable, there can be a deviation within the context of the presentinvention. Alternatively, the frequency of such address errors can bemonitored and if it exceeds a certain pre-determined level a deviationwithin the context of the present invention is present.

Alternatively, the optical response parameter RP can be an uncorrectableerror in an information sequence read from the optical carrier 1.Information decoding of encoded data from an optical carrier 1 normallyincludes an error correcting step (ECC), but for severe errors even thiscorrecting step cannot correct the errors and there may be a deviationwithin the context of the present invention. Alternatively, thefrequency of such uncorrectable errors can be monitored and if itexceeds a certain pre-determined level a deviation within the context ofthe present invention is present.

Alternatively, the optical response parameter RP can be an asymmetryvalue (often termed beta, β) measured from the optical response signals(RS) i.e. the HF signal from the optical carrier 1. If the asymmetryexceeds a certain level, preferably on a time-averaged basis, there canbe a deviation within the context of the present invention.

In one embodiment, the optical response signals RS can be compareddirectly with a reference value for the optical response signal itself.Thus, in that embodiment the optical response signal RS is equal to theoptical response parameter RP and accordingly the function ofsub-processor 53 is not needed. However, the sub-processor 53 willfacilitate easier and less complex comparison with reference values soas to obtain an indication of an unacceptably high level ofbirefringence. In particular, the asymmetry value of the HF signal is auseful indicator for the level of birefringence in the optical carrier1.

FIG. 4 is a simplified graph showing the modulated power outputP_(Laser) in the emitted radiation beam 5 of an irradiation source 4.The power of the beam 5 is periodically modulated in a square wavepattern. It comprises a plurality 80 of pulses, each pulse having aperiod being the reciprocal of the modulator frequency; 1/F1. In FIG. 4,each pulse has a high constant level 81 and low level 82 where theradiation power is zero or close to zero. By turning the radiation powercompletely off in the low level 82 and changing the modulator frequencyF1 it is possible by the teaching of the present invention tosubstantially decrease the optical feedback in the optical drive.

The modulator frequencies F1 and F2 have low and high limitations thatshould be mentioned. The modulator frequency cannot be chosen too lowotherwise data decoding is not possible as it is known from the Nyquistsampling theorem. On the other hand, the modulator frequency may not bechosen too high because of the wiring between the discrete modulator ordriver device 22 and the radiation source 4. Thus, a certain amount ofparasitic inductance (golden rule 10 nH/cm) should be taken into accountand it therefore becomes more and more difficult to switch e.g. a laser4 on to 50 mA and off to 0 mA at higher and higher frequencies F1 andF2. Finally, there are electromagnetic shielding (EMC) limitations withrespect to the rest of the optical drive and the surroundingenvironment. Thus, the emission can usually not exceed a specific level.Practically, the applied modulator frequencies F1 and F2 are in therange from 400 MHz to 500 MHz. However, the present invention canreadily be applied outside of this frequency interval once the generalprinciple of the invention is realized.

FIG. 5 is a schematic diagram of the optical path for an optical drivereading from a carrier 1 with no birefringence, i.e. Δn=0. Under realconditions there will usually not be exact zero birefringence, but insome cases it will be close to zero or alternatively effectively zerofor practical considerations. The optical path is similar to FIG. 1 withthe addition that a quarter wavelength plate (QWP) 16 is inserted infront of the carrier 1, and the focusing lens 7 in not shown for clarityin this Figure. In FIG. 5, the polarization state (linear or circular)and relative orientation (vertical or horizontal) is indicated atselected positions before and after the radiation beam 5 impinges on theoptical carrier 1. After passing the polarizing beam splitter (PBS) 6,the radiation 5 is linearly polarized in the vertical direction. Upontraversing the QWP 17, the beam 5 is circularly polarized and likewiseafter reflection from the carrier 1 but then circularly polarized in theopposite rotational direction. After the reflected radiation beam 8 haspassed the QWP 16, the polarization is again linear but now in thehorizontal direction so as to facilitate beam separation in thepolarizing beam splitter (PBS) 6. After the splitter 6, the reflectedradiation beam 8 is completely reflected towards the photo detectionmeans 10.

FIG. 6 is a schematic diagram of the optical path for an optical drivereading an optical carrier 1 with birefringence, i.e. Δn≠0. The opticalpath is similar to the optical path of FIG. 5, but due to thebirefringence of the optical carrier 1 the polarization of the reflectedradiation 8 will be distorted relative to the ideal situation shown inFIG. 5. Such a distortion can be an additional rotation of thepolarization state. In general, it can be stated that birefringenceintroduces wave front error (usually expressed in nanometers). A furthereffect is the so-called optical leaking of power into the substrate ofthe optical carrier 1, i.e. a lowering of the reflectivity. The effectof the birefringence of the optical carrier 1 is seen at the polarizingbeam splitter (PBS) 6, where the splitter 6 is unable to completelyreflect the radiation 8 towards the photo detection means 10. Rather,the radiation 8 is split into two directions; one direction towards thephoto detection means and another direction back towards the radiationsource 4. The latter component represents the optical feedback 15 thatis undesirable due to the negative influence on the emitted radiation 5from the radiation source 4 as explained in connection with FIG. 2.Furthermore, the optical feedback 15 represents a power loss becausethis component cannot of course be measured by the photo detection means10, and hence this results in lower signal intensity in the photodetection means 10.

FIG. 7 is a graph showing the birefringence across an optical carrier 1of the DVD format. The birefringence is the relative birefringencemeasured in nanometers (nm) as indicated on the vertical axis on thegraph of FIG. 7, and on the horizontal axis the radial position (mm) onthe carrier 1 is indicated between 23 mm and 58 mm. The upper curve andlower curve of the birefringence indicate measured maximum and minimumvalues, respectively, according to ECMA standard for BD disks. Thebirefringence is above 100 nm for a radius below approximately 52 mm.The 100 nm is the upper specification limit (USL) marked by thehorizontal line in the graph. Thus, this optical carrier 1 is for alarge portion of the carrier out of the specification with respect tobirefringence. Such an out-of-spec carrier or disk, also known in theart as a “horror disc”, is very likely to cause a fatal reading errorunless the optical drive is provided with a compensational mechanismtherefore. Indeed, the present invention provides an optical drive thatenables an optical drive to compensate for such a birefringence on theoptical carrier 1 in a very effective and cost-effective manner.

FIG. 8 is a flow-chart of a method according to the invention foroperating an optical drive for reading information from an opticalcarrier 1, the method comprising the steps:

emitting a radiation beam 5 having a read power level 80 and 81modulated with a first modulator frequency F1, said radiation beam beingoptically arranged to impinge on the optical carrier 1,

guiding by polarizing beam splitting means (PBS) 6 a reflected beam 8from the carrier 1 towards photo detection means 10, said photodetection means being capable of outputting optical response signals RS,the two first steps is taken to be a part of the START box in the flowchart,

processing said optical response signals RS received from the photodetection means into one or more optical response parameters RP as shownin second box “RS, RP”,

monitoring if an optical response parameter RP is deviating from apre-determined optical response parameter reference value RP_ref asindicated by the decision box “RP_ref?”, said deviation being indicativeof an optical feedback 15 from the beam splitting means (PBS; 6) towardsto the radiation source 4, and if a deviation occurs (marked by thesymbol A, Greek delta) changing the first modulator frequency F1 of theradiation beam 5 to a second modulator frequency F2, indicated by thebox “F1→F2”, so as to decrease the optical feedback 15 shown FIGS. 3 and6. The closed loop feedback is limited to certain number of loops toensure a stopping of the loop.

Finally, if no deviation is present between an optical responseparameter RP and a pre-determined optical response parameter referencevalue RP_ref the method can proceed to the box READ.

Although the present invention has been described in connection with thespecified embodiments, it is not intended to be limited to the specificform set forth herein. Rather, the scope of the present invention islimited only by the accompanying claims. In the claims, the term“comprising” does not exclude the presence of other elements or steps.Additionally, although individual features may be included in differentclaims, these may possibly be advantageously combined, and the inclusionin different claims does not imply that a combination of features is notfeasible and/or advantageous. In addition, singular references do notexclude a plurality. Thus, references to “a”, “an”, “first”, “second”etc. do not preclude a plurality. Furthermore, reference signs in theclaims shall not be construed as limiting the scope.

1. An optical drive for reading information from an associated opticalcarrier (1), the optical drive comprising: a radiation source (4)capable of emitting a radiation beam (5) having a read power level (80,81) modulated with a first modulator frequency (F1), said radiation beambeing optically arranged to impinge on the associated optical carrier(1), polarizing beam splitting means (PBS; 6) arranged for guiding areflected beam (8) from the carrier (1) towards photo detection means(10), said photo detection means being capable of outputting opticalresponse signals (RS), and processing means (50, 51, 52, 53) forprocessing said optical response signals (RS) received from the photodetection means (10) into one or more optical response parameters (RP),said processing means comprising comparison means (51) arranged formonitoring if an optical response parameter (RP) is deviating from apre-determined optical response parameter reference value (RP_ref), saiddeviation being indicative of an optical feedback (15) from the beamsplitting means (PBS; 6) towards to the radiation source (4), theprocessing means (52) further being adapted for changing the firstmodulator frequency (F1) of the radiation beam (5) to a second modulatorfrequency (F2) so as to decrease the optical feedback (15).
 2. Anoptical drive according to claim 1, wherein the optical response signals(RS) are representing information read from the optical carrier (1). 3.An optical drive according to claim 2, wherein the optical responseparameter (RP) is chosen from the group consisting of: an addresslocating a position on the optical carrier (1), an uncorrectable errorin an information sequence read from the optical carrier (1), and anasymmetry value (beta) measured from the optical response signals (RS)from the optical carrier (1).
 4. An optical drive according to claim 1further comprising an auxiliary radiation source adapted for emitting anauxiliary radiation beam, said beam being optically arranged forimpinging on the optical carrier (1) and resulting in a reflected beamtowards the photo detection means (10).
 5. An optical drive according toclaim 1 further comprising auxiliary photo detection means for detectionof reflected radiation representing information read from the opticalcarrier (1).
 6. An optical drive according to claim 1, wherein thesecond modulator frequency (F2) is larger than the first modulatorfrequency (F1).
 7. An optical drive according to claim 6, wherein theprocessing means (50) is arranged for increasing the modulator frequency(F1; F2) in a feedback loop so to minimize the deviation of an opticalresponse parameter (RP) from a pre-determined optical response parameterreference value (RP_ref).
 8. An optical drive according to claim 1,wherein the radiation beam (5) has a substantially square-wave powerprofile.
 9. An optical drive according to claim 1, wherein the radiationbeam (5) is optically arranged for passing the polarizing beam splittingmeans (PBS; 6) on the optical path towards the optical carrier (1). 10.Processing (50, 51, 52, 53) means for controlling an associated opticaldrive for reading information from an associated optical carrier (1),the associated optical drive comprising: a radiation source (4) capableof emitting a radiation beam (5) having a read power level (80, 81)modulated with a first modulator frequency (F1), said radiation beambeing optically arranged to impinge on the associated optical carrier(1), and polarizing beam splitting means (PBS; 6) arranged for guiding areflected beam (8) from the carrier towards photo detection means (10),said photo detection means being capable of outputting optical responsesignals (RS), the processing means being adapted for processing saidoptical response signals (RS) received from the photo detection meansinto one or more optical response parameters (RP), said processing meanscomprising comparison means (51) arranged for monitoring if an opticalresponse parameter (RP) is deviating from a pre-determined opticalresponse parameter reference value (RP_ref), said deviation beingindicative of an optical feedback (15) from the beam splitting means(PBS; 6) towards to the radiation source (4), the processing means (52)further being adapted for changing the first modulator frequency (F1) ofthe radiation beam (5) to a second modulator frequency (F2) so as todecrease the optical feedback (15).
 11. Method for operating an opticaldrive for reading information from an optical carrier (1), the methodcomprising the steps: emitting a radiation beam (5) having a read powerlevel (80, 81) modulated with a first modulator frequency (F1), saidradiation beam being optically arranged to impinge on the opticalcarrier (1), guiding by polarizing beam splitting means (PBS; 6) areflected beam (8) from the carrier (1) towards photo detection means(10), said photo detection means being capable of outputting opticalresponse signals (RS), processing said optical response signals (RS)received from the photo detection means (10) into one or more opticalresponse parameters (RP), monitoring if an optical response parameter(RP) is deviating from a pre-determined optical response parameterreference value (RP_ref), said deviation being indicative of an opticalfeedback (15) from the beam splitting means (PBS; 6) towards to theradiation source (4), and changing the first modulator frequency (F1) ofthe radiation beam (5) to a second modulator frequency (F2) so as todecrease the optical feedback (15).
 12. A computer program product beingadapted to enable a computer system comprising at least one computerhaving data storage means associated therewith to operate an opticaldrive according to claim 11.